A charge ratio digitizer of simple construction is one of the wedge and strip type. A digitizer of this type comprises a digitizing tablet and a cursor or stylus movable thereon, wherein an electrical field coupling is utilized to ascertain the position of the cursor or stylus relative to the digitizer tablet. The coordinate system is typically the x-y coordinate system. This type of digitizer is disclosed in U.S. Pat. No. 4,705,919.
The cursor or stylus comprises a housing movable on the surface of the digitizer tablet, the housing containing a coupling electrode. In one embodiment an input signal is applied to the coupling electrode of the cursor or stylus. The coupling electrode field electrically couples to wedge (sawtooth) and strip patterned electrodes embedded in the digitizer tablet. One of the wedge or strip patterned electrodes is used to ascertain the x-axis position of the cursor and the other is used to ascertain the y-axis position of the cursor. The wedge and strip patterned electrodes are conveniently provided in a coplanar arrangement on a printed circuit board and are covered with a layer of dielectric material to provide a smooth tablet surface. In the embodiments where the coupling electrode of the cursor or stylus is driven, the signals induced in the patterned electrodes are processed electronically to ascertain the position of the cursor or stylus relative to the tablet.
In accordance with more specific aspects of the wedge-and-strip type of digitizer, the digitizer tablet has three patterned electrodes which produce an x-axis (X) signal, a y-axis (Y) signal and a balance (B) signal. The X electrode comprises a plurality of conductive rectangular strips disposed vertically and arranged at equal intervals and in order of increasing width from one side to the other side of the tablet, e.g., increasing in width from the left to the right side of the tablet. The Y electrode comprises a plurality of uniform conductive wedges or sawtooths also disposed vertically and arranged at equal intervals with the Y wedges interleaved with the X strips. Movement of the cursor or stylus in the x-axis direction has a negligible effect on the cursor (stylus) coupling with the Y electrode and movement of the cursor (stylus) in the y-axis direction has a negligible effect on the cursor (stylus) coupling with the X electrode. The balance (hereinafter "B") electrode substantially covers the area of the digitizer tablet not covered by the X and Y electrodes, except for interelectrode spacing therebetween.
The coupling electrode located in the cursor (stylus) housing and movable on the surface of the digitizing tablet surface is connected to an oscillator whose frequency typically is in the range of 10 kHz to 1 MHz. The coupling electrode induces electrical signals in the X, Y and B electrodes.
The surface area of the X electrode coupled with the coupling electrode increases as the cursor or stylus is moved or positioned toward the one side of the tablet where the strips are wider, and hence more electrical signal is induced in the X electrode. Similarly, the surface area of the Y electrode coupled with the coupling electrode increases as the cursor or stylus is moved to a region on the tablet where the wedges are wider, and hence more electrical signal is induced in the Y electrode. The X, Y and B electrodes completely cover the area of the tablet except for narrow spacing between the electrodes. It is therefore expected that the sum of the signals on the X, Y and B electrodes will be independent of cursor (stylus) position. This sum signal depends primarily upon the amplitude of the oscillator signal, the proximity of the coupling electrode to the X, Y and B electrodes, and the dielectric constant of the material between the coupling electrode and the X, Y and B electrodes.
Based upon mathematical and electrical concepts, the x coordinate of the cursor or stylus is proportional to the X electrode signal divided by the sum of the X, Y, and B electrode signals. Similarly the y coordinate of the cursor or stylus is expected to be proportional to the Y electrode signal divided by the sum of the X, Y and B electrode signals. In theory, this ratiometric technique should enable accurate measurement of the cursor (stylus) position and be independent of the proximity of the cursor or stylus to the digitizer tablet and the dielectric therebetween.
In the preferred embodiment of wedge-and-strip digitizer, the functions of the coupling electrode and the patterned electrodes are reversed, i.e. input signals are successively applied to the X, Y and B electrodes, and induced signals are output from the coupling electrode in response thereto.
The prior art wedge-and-strip system is disadvantageous in that distortion of the tablet size results from the spacing between the X, Y and B electrodes in the digitizer tablet. This phenomenon will hereinafter be referred to as "resolution distortion". Spacing between the electrodes in the pattern cannot be eliminated because it is necessary to electrically isolate one electrode from the other. Mathematical relationships upon which wedge/strip digitizer design is based require that there be no space between electrodes in order to provide accurate data. However, as indicated, this is a practical manufacturing impossibility. When spacing between electrodes is precisely known and is uniform over the entire pattern, then a mathematical adjustment can be made so that distortions in the output pattern due to electrode spacing can be compensated for.
A further disadvantage of the prior art wedge-and-strip digitizer is that the reading includes a value corresponding to environmental and inherent noise as well as the value produced by electrostatic induction. The environmental noise is attributable to the fact that the tablet acts as an antenna. The inherent noise is caused by undesired internal inductive coupling between components. In addition, the reading includes a value corresponding to an offset representing the difference between the reference potentials of the integrator and comparator.
What is needed is a digitizer construction which reliably and economically provides compensation for the unavoidable spacings between tablet electrodes and for the background effects, i.e. the noise and the offset.